1. Field of the Invention
The present invention relates to an adhesive for polarizing plate. The present invention further relates to a polarizing plate and manufacturing method thereof using the adhesive for polarizing plate. The polarizing plate alone or an optical film obtained by laminating the polarizing plate can constitute an image display such as a liquid crystal display, an organic EL display or PDP.
2. Description of the Related Art
In the liquid crystal displays, for example, it is indispensable to dispose polarizers on both sides of a glass substrate providing a surface of a liquid crystal panel according to an image formation scheme adopted in the display. A polarizer is generally obtained in a procedure in which a polyvinyl alcohol-based film is dyed with a dichroic material such as iodine, thereafter, the film is crosslinked with a crosslinking agent and then, mono-axially stretched to thereby form a film. Since the polarizer is manufactured by stretching, it is easy to shrink. Since a polyvinyl alcohol-based film comprises a hydrophilic polymer, the film is very easily deformed especially in a humidified condition. Since the film itself is weak in mechanical strength, there has been a problem that the film is torn. Hence, adopted is a reinforced polarizing plate manufactured in a procedure in which a transparent protective film or transparent protective films each made from triacetyl cellulose or the like are adhered to on one side or both sides of a polarizer. The polarizing plate is manufactured by adhere the transparent protective film onto a polarizer using an adhesive.
In recent years, the range of uses of liquid crystal displays has been increased and extended to cover from portable terminals to large screen home TVs, and their standards have been defined for each application. In particular, portable terminal applications, which must be portable for users, strongly demand durability. For example, polarizing plates are required to have water resistance to such an extent that their properties and form will not change even under humidified conditions, further humidified conditions at high temperature such as conditions causing dew condensation.
As mentioned above, a polarizer may be reinforced in strength with a transparent protective film to form a polarizing plate before use. As an adhesive for polarizing plate used in adhesion of the polarizer and the transparent protective film to each other, an aqueous type adhesive is conventionally preferable and, for example, a polyvinyl alcohol-based adhesive obtained by mixing a crosslinking agent into a polyvinyl alcohol aqueous solution has been employed. A polyvinyl alcohol-based adhesive may cause peeling at the interface between a polarizer and a transparent protective film in a humidified environment. This is considered because a polyvinyl alcohol-based resin, which is a main component of the adhesive, is a water-soluble polymer and a possibility of dissolution of an adhesive occurs in a situation of dewing. In order to cope with the problem, a proposal has been offered of an adhesive for polarizing plate containing a polyvinyl alcohol-based resin having an acetoacetyl group, and a crosslinking agent (see JP-A No.7-198945).
On the other hand, when a polarizer and a transparent protective film are laminated together with the aqueous polyvinyl alcohol-based adhesive interposed therebetween in the process of preparing a polarizing plate, it is desired to improve the wettability of the transparent protective film by the adhesive. For example, therefore, it is proposed that the transparent protective film should be subjected to activation treatment such as corona or plasma treatment (see JP-A No. 2006-178191). In some cases, however, even after the transparent protective film is subjected to activation treatment, the activation is insufficient so that sufficient wettability cannot be obtained, depending on the type of the transparent protective film. In order to improve wettability, it is also proposed that an additive such as a fluoro surfactant or any other surfactant, a specific acrylic resin, or a silicone leveling agent or any other leveling agent should be added to the adhesive. However, mixing the additive into the adhesive can undesirably reduce the adhesion (particularly water-resistant adhesion) or lead to the production of polarizing plates with degraded optical properties.
The polarizing plate may be manufactured by a method (precoating method) including the steps of applying the aqueous polyvinyl alcohol-based adhesive to a transparent protective film and then laminating a polarizer to the transparent protective film after or without drying or by another method (simultaneous coating method) including the step of laminating a transparent protective film to a polarizer, while interposing the adhesive between them by coating. Since wettability is required to be good in the precoating method, however, polarizing plates obtained by the precoating method have a problem in which they can suffer from visible unevenness.